The present invention relates to video timing, and more particularly to a video timing display for multi-rate systems to allow a user to quickly, easily and precisely measure the timing difference between video signals of different rates.
The traditional method for checking timing between video signals is to use a waveform display and a vectorscope. The signal of interest is used as the input and a reference signal is applied to an “external” reference port. First one uses a field rate sweep to check vertical timing. Next one uses a line rate sweep to check horizontal timing. Finally in a composite system one uses an SCH (subcarrier-to-horizontal) mode to insure color frames are aligned and to see the timing with enough precision to match the subcarrier phases.
This traditional method is time consuming, difficult and fraught with possibilities for error. To see the horizontal and vertical timing one must shift the waveform and zoom in on features. As the system is adjusted to the desired timing, the display must be manually positioned to follow. Checking the horizontal timing may be imprecise because of the slow rise times of the video sync pulses. Also not all monitors display SCH correctly when in external reference mode so alignment of color frames is problematic. Finally one may have to switch between internal and external reference mode, or switch between the inputs, to compare the timing of the signals.
An alternative method of timing offset determination is to look at a picture relative to the external reference. This has the advantage of showing both the horizontal and vertical offset at the same time. Unfortunately it is difficult to be precise with this method and it is not well suited for checking color frame alignment.
With traditional methods timing reference measurement requires that the line and frame rate of the input be an integer multiple of the reference rates. If this requirement is not met, the measurement results are non-deterministic, and thus cannot be used. One example, as shown in FIG. 1, are the timing differences between a 24 Hz video relative to a 30 Hz video as a reference. For each frame of the 30 Hz reference there is one of four possible delays—d1, d2, d3, d4—to the next frame of the 24 Hz input. The pattern repeats after five frames of the 30 Hz input signal or four frames of the 24 Hz signal. Each group of frames comprises a “superframe” for each signal—five frames for the 30 Hz signal and four frames for the 24 Hz signal. If one simply divides the 30 Hz signal by four, then one is effectively choosing one of these phases and one will randomly end up with any of these four delays as the detected timing. Also some video signal combinations with different rates have different numbers of possible phases. Other common standards have either two or five such ambiguous phases, for example.
SMPTE 318M provides one possible way out of this ambiguity for some combinations. The SMPTE 318M standard specifies an extension to carry a ten-field sequence identification on an NTSC black signal to be used as a reference. The ten-field signal is five frames long, so it effectively identifies which of the five delays is the “correct” one and eliminates the other four. Unfortunately many video signals do not carry the SMPTE 318M ten-field identification flags, so other methods are still needed.
Another method looks at the multiple possible phases and selects the smallest offset. The waveforms are then displayed as if that was the correct timing. This works in many cases, but it does not illustrate to a user what choice was made, nor does it allow for locking to a particular phase of one of the video signals as a reference and measuring timing for multiple video signals, so it really does not provide for timing one video signal to another of the same rate via a known relationship to the reference at a different rate.
What is desired is a mechanism and display to allow a user to quickly and easily measure the timing difference between video signals of different rates.